wgs84 variants

Hello,

We use a very nice gps post processing package called GrafNav. I recently
had some (very belated) training in it and was surprised to learn that
wgs84 and nad83 aren't really coincident anymore, and haven't been for some
time. Its hard to find any info about it but I have found this:
http://www.geod.nrcan.gc.ca/faq_e.php#19

Now I am on the hunt for a package or library that supports the different
versions of wgs84. I don't think its as simple as defining a new datum in
your favorite tool, since the datum definitions are always relative to
wgs84!

The Geographic Calculator, as of 6.3, still considers them the same, as
does GeoTrans 2.4.

I'm looking for more info on this topic. It could very well be that I'm
making a mountain out of a molehill, but we spend a lot of money, time and
effort getting our positions as accurate as possible, and I need to know
I've eliminated these sorts of static errors right off the bat.

Currently we use novatel oem4 dual frequency receivers for rover and base,
positioning the base from CORS stations whenever possible. Grafnav
typically reports the accuracy of our rover positions to be about 20 cm.
horizontally

cheers

Marc Pelletier

Goldak Airborne Surveys

Marc Pelletier wrote:


Look at: http://www.ngs.noaa.gov/PC_PROD/HTDP/

It will give you some idea about NAD83 and (various) WGS84 datum errors.
To do any type of high accuracy GPS processing you need to understand these
differences. Whether or not it really matters for the end product of the work
you do depends. There may be other greater errors involved, but at a basic
professional level you should really understand this. Work through a few
examples. Depending on what you may be mixing up and the baseline
lengths involved the errors could be sub-millimeter, cm, or meter level.

Thanks John and others. The HTPD program seems to be what I was looking
for.

Another private responder also pointed me at some very nice documentation
at http://nauticalcharts.noaa.gov/csdl/Vdatum_data/VDatum106.pdf


Sections 7 & 8 have a nice description of the issues.

Thanks again,

Marc

Marc
Try this link and read the paper on WGS to GDA. To answer you question
replace GDA - Australian datum with NAD.
http://www.ga.gov.au/geodesy/reports/datum_transformations/

Cheers

Marc Pelletier wrote:

My understanding is that the Australians are living on a fast moving
crust. I think they are traveling NNE at about 6cms per year (someone
should check this for me though). This means that in 10 years they
have moved more than half a metre towards the equator. Because of this
any transformations between WGS84 and their own reference system
(GDA94) are time dependent.

We in the UK are traveling East as the Atlantic Ocean is getting wider
by about 1cm per year . Europeans use the EUREF frame of reference -
ETRS89 - You can see how it changes by year at
http://lareg.ensg.ign.fr/EUREF/

You need to understand which tectonic plate you are on and how this is
moving with respect to ITRS.

I wonder how it works in parts of California or Vancouver Island which
are close to major boundaries between plates. Do they have a different
frame of reference on each side of the fault line?

Alan Murphy

On 12 Dec 2006 21:16:32 -0800, danny.galbraith@ga.gov.au wrote: